A radio device includes a high-frequency amplifier and an antenna that is connected to the output terminal of the high-frequency amplifier with a transmission line. When a radio signal is transmitted with a large power while the high-frequency amplifier, the transmission line, and the antenna are not matched, the devices of the high-frequency amplifier may be damaged due to the “reflected wave” from the antenna. In order to avoid the damage of the devices of the amplifier, conventional radio devices measures the voltage wave standing ratio (VSWR) that is the ratio between the “forward wave” and the “reflected wave” and implement control to stop the output of the high-frequency amplifier when it is determined, according to the measured VSWR, that there is a risk that the devices of the amplifier would be damaged. The “forward wave” corresponds to the signal transmitted by the radio device and the “reflected wave” corresponds to the signal that is the transmitted wave reflected by the antenna.
In the feedback path for measuring reflected waves from the antenna, there would be a received signal that is a signal from a different station received by the antenna (that can be referred to as a “different station signal” below) in addition to the reflected wave from the antenna. When the different station signal is mixed into the feedback signal, the accuracy of the VSWR lowers.
Thus, conventionally, in the feedback path, only the signal components of the “transmission band” are extracted using a band-pass filter and, using the extracted signal components, the VSWR is measured. The radio device performs upsampling on the transmitted signal of the “baseband signal band” to obtain a transmitted signal of the “transmission band”. The radio device further performs radio processing, such as digital-analog conversion, upconversion and amplification, on the transmitted signal of the “transmission band” and then transmits the resulting radio signal. The whole band including at least one transmission band can be referred to as the “system band”. The “baseband signal band” can be referred to as the “baseband processing rate”, “baseband processing sampling frequency”, or “baseband processing data rate”. The “transmission band” can be referred to as the “transmission processing rate”, “transmission processing sampling frequency”, or “transmission processing data rate”. Conventional examples are described in Japanese Laid-open Patent Publication No. 2013-165418, Japanese Laid-open Patent Publication No. 2007-281943 and Japanese Laid-open Patent Publication No. 2007-282238
The conventional technology, however, measures the VSWR based on the signal components of the whole transmission band, which may increase the costs of operations for the measurement.
When a radio device that is mounted on a terminal that supports a system band including several transmission bands (e.g. a dual terminal) measures the VSWR based on the signal components of the whole system band, the costs of operations for the measurement may further increase. For example, measuring the VSWR has repeated operations according to the cycle of integration of a relatively short time (e.g. 10 sec.). For this reason, when the VSWR is measured based on the signal components of the whole system band, the load of operations performed by the radio device increases as the system band broadens.